Final answer:
The man promises the sorceress his firstborn child when she catches him stealing rapunzel from her garden.
Step-by-step explanation:
In the popular fairy tale Rapunzel, when the man is caught stealing rapunzel (a plant) from the sorceress's garden, he promises her his firstborn child. This event marks the beginning of the hardships and adventures faced by the characters in the story, spiraling into a classic narrative of confinement, love, and eventual freedom.